Coordination of tetradentate X2N2 (X = P, S, O) ligands to iron(ii) metal center and catalytic application in the transfer hydrogenation of ketones

Article abstract of DOI:10.1039/b816439h

A series of mixed tetradentate ligands associating two iminophosphorane moieties with two phosphino, thiophosphino, or phosphine oxide groups (labelled 2, 3, and 4 respectively) have been prepared from the corresponding aminophosphonium derivatives. Their

Full text of DOI:10.1039/b816439h

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PAPER
www.rsc.org/dalton | Dalton Transactions
Coordination of tetradentate X2N2 (X = P, S, O) ligands to iron(II) metal
center and catalytic application in the transfer hydrogenation of ketones†
Antoine Buchard, Hadrien Heuclin, Audrey Auffrant, Xavier F. Le Goff and Pascal Le Floch*
Received 19th September 2008, Accepted 26th November 2008
First published as an Advance Article on the web 27th January 2009
DOI: 10.1039/b816439h
A series of mixed tetradentate ligands associating two iminophosphorane moieties with two phosphino,
thiophosphino, or phosphine oxide groups (labelled 2, 3, and 4 respectively) have been prepared from
the corresponding aminophosphonium derivatives. Their coordination to the iron dichloride metal
fragment was achieved using the [FeCl2(THF)1.5] precursor leading to [(P2N2)FeCl2] (5), [(S2N2)FeCl2]
(6), and [(O2N2)FeCl2] (7). These complexes were shown to be paramagnetic. Moreover, those ligands
can act as bi, tri or tetradentate, as evidenced by X-ray structure analysis of the complexes, depending
on the nature of the pendant PY coordinating ligand (Y = lone pair, S, O). Indeed, while only one
phosphorus atom is coordinated to iron in 5 (PNN coordination), no thiophosphine moiety is
connected to Fe in 6 (NN coordination), whereas both phosphine oxide arms are linked to the metal in
7 (ONNO) coordination. For ligand 2, coordination reactions were also performed with a
non-chlorinated iron precursor (either [Fe(CH3CN)6](BF4)2 or [Fe(H2O)6](BF4)2) leading to complexes
8 and 9 depending on the reaction conditions. The different iron(II) complexes 5–9 were tested in
catalytic transfer hydrogenation of acetophenone, and were found to be efficient for reactions carried
out at 82 ◦C.
Introduction
Development of efficient homogeneous catalysts able to achieve
transformations at low economical and environmental cost is
fundamental from an industrial point of view. In this context,
the use of iron-based complexes is particularly attractive due to
the availability and the cost of this metal. Thus in the last decade,
a lot of effort was devoted to the development of easily available
and cheap iron based catalysts featuring nitrogen based ligands. In
particular, bis(iminopyridine) iron(II) complexes were shown to be
excellent ethylene oligomerisation1,2 or polymerisation catalysts.3,4
More recently, Chirik and coworkers successfully reduced such
(N,N,N) iron(II) precursors under nitrogen atmosphere to obtain
Chart 1 Tetradentate ligands.
efficient iron(0) precatalysts for the hydrogenation and hydrosily-
lation of olefins and alkynes.5 The same group also reported the
reduction of a-diimine iron(II) complexes in presence of alkynes or
alkenes to prepare (N,N)Fe(0) complexes that are able to perform
the catalytic hydrogenation of 1-hexene.6
this context, we recently developed the synthesis of the phosphorus
analog of such tetradentate ligand bearing two phosphine and
However, while tetradentate mixed (P2N2) ligands (1, Chart 1)
combining two “soft” phosphines and two “hard” nitrogen atoms,
either imine or amide groups, were used with great success in
palladium or ruthenium chemistry,7 their coordination to iron was
much less investigated. Whereas Gao and coworkers described the
synthesis and structure of (P2N2)Fe(II) complexes in 1996,8 the
involvement of such complexes in catalytic processes was only
reported this year by Morris’s group,9 who performed the first
asymmetric hydrogenation of polar bonds using iron complexes. In
=
two iminophosphorane (P N) moieties (2, Chart 1), having in
mind that iminophosphoranes exhibit different electronic and
steric properties than classical imines. Indeed, iminophosphoranes
have no p-accepting ability because of the absence of a real
p-system, and behave as strong p-donor ligands due to the
presence of two lone pairs on the nitrogen atom. We studied the
coordination of tetradentate ligand 2 to palladium and nickel
centres, and demonstrated the potential of the corresponding
complexes for Suzuki–Miyaura reactions in a biphasic medium.10
Pursuing our investigations on this ligand, we report here our
study concerning the coordination of ligands 2, 3, and 4, which are
the corresponding sulfated and oxygenated derivatives respectively
(Chart 1), to iron(II) metal centres. Catalytic activity of these
complexes in the transfer hydrogenation of ketones is also
presented and discussed.
Laboratoire «He´te´roe´le´ments et Coordination», Ecole Polytechnique,
CNRS, Route de Saclay, 91128, Palaiseau, France. E-mail: pascal.
lefloch@polytechnique.edu; Fax: +33 169334440
† CCDC reference numbers 702897–702900. For crystallographic data in
CIF or other electronic format see DOI: 10.1039/b816439h
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Results and discussion
A
Ligand syntheses and coordination studies
Iminophosphorane-based ligands are, in most cases, straightfor-
wardly prepared by the Staudinger reaction involving the thermal
reaction of azides and phosphines.11 However, with this method-
ology, the substitution pattern at the nitrogen atom is generally
restricted to trimethylsilyl, bulky aryl, sulfonate or phosphonate
groups due to the poor availability of functional azides. Thus, we
employed a different strategy based on the Kirsanov reaction.12
This approach consists in the bromination of the phosphine
followed by reaction of the formed bromophosphonium with
a primary amine in the presence of a base to yield finally the
corresponding aminophosphonium, which is perfectly air and
moisture stable. Moreover, the use of diamines opens the way to
iminophosphorane ligands which would be very difficult to access
using azides.
As already described,10 compound 2-(HCl)2 (Scheme 1) was
readily obtained by reaction of bromine with dppm (bis-
(diphenylphosphino)methane), followed by the addition of one
equivalent of ethylenediamine (half the equivalent behaving as
a nucleophile, the second as a base). In order to avoid any
scrambling of the bromide counteranions with the chlorides of
iron precursors, the dication was isolated as the chloride derivative
in 72% yield after washing with saturated brine solution. The
tetradentate phosphine-iminophosphorane ligand (2) will then be
generated in situ before coordination experiments by addition of
two equivalents of base.
Scheme 2 Synthesis of 3-(HCl)2 and 4-(HCl)2.
analysis. Contrary to what is observed in the case of the sulfide
derivative 3-(HCl)2, the 31P{H} NMR spectrum of 4-(HCl)2
exhibits a characteristic AB spin system pattern centered at
d(CDCl3) 44.6 ppm (2JPP = 8 Hz) for the phosphonium group
and at 28.6 ppm (2JPP = 8 Hz) for the phosphine oxide moiety.
As described for the synthesis of 2 from 2-(HCl)2,10 the
iminophosphorane based ligands 3 and 4 were prepared by
deprotonation of the corresponding dications using 2 equivalents
of MeLi in toluene at low temperature (Scheme 3). The 31P{H}
NMR data of 3 and 4 were found to be very similar, both exhibiting
a characteristic AB spin system pattern centered at d(d8-toluene) =
38.0 (2JPP = 26.0 Hz) and 38.7 (2JPP = 12 Hz), for the thio- or oxo-
phosphino group respectively and at 27.9 ppm in 3 and at 33.2 ppm
in 4 for the iminophosphorane moiety.
Scheme 1 Synthesis of 2-(HCl)2.
The thio and oxo derivatives have been prepared by a similar
strategy involving the corresponding cations 3-(HCl)2 and 4-
(HCl)2 respectively. Reaction of the dicationic 2-(HCl)2 with a
stoichiometric amount of S8 in dichloromethane at 40 ◦C induced
within 30 min the precipitation of the sulfide derivative 3-(HCl)2
as a white powder (Scheme 2, path a). 3-(HCl)2 was obtained in
excellent yield (92%) and was characterized by multinuclear NMR
techniques and elemental analysis. 31P{H} NMR spectrum show
two singlets at 37.0 and 44.7 ppm corresponding respectively to the
thiophosphinoyl and aminophosphonium groups. Interestingly,
no coupling between the two non-equivalent phosphorus could be
measured. The phosphine oxide derivative 4-(HCl)2 was prepared
by oxidation of the phosphino groups of the dicationic 2-(HCl)2
by hydrogen peroxide (1.5 equiv.) in dichloromethane at room
temperature (Scheme 2, path b). After washing with water then
drying, 4-(HCl)2 was isolated as a white powder in 85% yield. It
was characterized by multinuclear NMR techniques and elemental
Scheme 3 Preparation of iminophosphorane-based ligands.
These ligands could be isolated as air and moisture sensitive
white powders after removal of precipitated LiCl salts and
evaporation of solvents. Because of their sensitivity 3 and 4 were
only characterized by multinuclear NMR techniques.
As previously explained, coordination experiments were di-
rectly conducted from the dications, without isolating the free
ligands 2–4. Thus, their deprotonation was achieved in THF
at low temperature, followed by the addition of [FeCl2(THF)1.5]
(Scheme 4). The coordination reaction could be monitored by
31P{H} NMR showing the total disappearance of the AB spin
system pattern corresponding to the free ligands 2–4. In each case
the coordination was accompanied by a marked colour change of
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Fig. 1 Molecular structure of complex 5. Hydrogen atoms and solvent
˚
molecules have been omitted for clarity. Selected distances (A) and angles
(◦): P4–Fe = 5.296; N1–Fe = 2.074(2); N2–Fe = 2.116(2); P2–Fe =
2.676(8); Fe–Cl2 = 2.406(7); Fe–Cl1 = 2.350(7); P1–N1 = 1.593(2);
P3–N2 = 1.581(3); Cl1–Fe–Cl2 = 123.84(3); Cl1–Fe–N2 = 96.26(8);
Cl2–Fe–N2 = 100.98(8); Cl1–Fe–N1 = 116.28(7); Cl2–Fe–N1 = 119.06(7);
Cl1–Fe–P2 = 98.27(3); Cl2–Fe–P2 = 85.13(2); P2–Fe–P4 = 145.64;
N1–Fe–P2 = 77.30(7); N1–Fe–N2 = 80.8(1); N2–Fe–P2 = 157.38(7);
Fe–P2–N1–N2 = 4.52; P2–N1–N2–P4 = 6.34; P2–P1–P3–P4 = 24.70;
Fe–N1–Cl2–Cl1 = 6.04.
Scheme 4 Coordination with [FeCl2(THF)1.5].
the reaction mixture which became yellow upon forming 5, brown
upon forming 6 and 7.
After completion of the reaction, THF was evaporated in vacuo,
dichloromethane was introduced, inducing the precipitation of
the LiCl salts which were eliminated by filtration. After removal
of CH2Cl2, and washing with hexanes, complexes 5 and 6 were
isolated as air and moisture sensitive yellow and brown powders
respectively, whereas 7, which appeared a lot more air and moisture
sensitive was isolated as a yellow brown powder. The latter indeed
decomposes when handled in air even for a few seconds as
evidenced by an instant colour change to red.
The 31P{H} NMR spectra of complexes 5–7 solutions are
poorly definite, because of their paramagnetism. Nevertheless,
significant differences can be observed depending on the ligand
used. Indeed, while for 5 and 6, after long acquisition time, a
broad singlet can be observed at d(THF) = -27 ppm and 57 ppm
respectively, which may correspond to uncoordinated phosphine
and thiophosphine arms respectively, no signal could be seen for
solutions of complex 7, which might indicate the coordination of
both iminophosphorane and phosphine oxide moieties.
observed paramagnetism. The broad signal observed in 31P NMR
for the free phosphine moiety also suggests that, in solution, the
phosphine groups are in permanent exchange.
A view of complex 6 is shown in Fig. 2. In this case, the
geometry around the iron(II) centre is tetrahedral since only
the two iminophosphorane arms and two chloride atoms are
coordinated, all the angles around iron being relatively close
(angles : Cl1–Fe–Cl2 = 118.42(3)◦; Cl1–Fe–N2 = 113.88(6)◦; Cl2–
Fe–N2 = 110.46(6)◦; Cl1–Fe–N1 = 111.26(6)◦). On the other
hand, the Cl–Fe bond distances were measured at 2.301(7) and
˚
2.312(7) A.
To confirm the paramagnetic nature of complexes 5–7, the
magnetic moments of these complexes were determined using the
method of Evans in CDCl3 at 20 ◦C employing tetramethylsilane
as reference. These measurements gave values of 4.36 mB for 5,
3.87 mB for 6, and 4.03 mB for 7, which are in the range of those
determined for other iron(II) complexes.1,3,14
However, definitive evidences concerning the structures of
complexes 5–7 were furnished by X-ray crystal structure analysis
of these complexes. Suitable crystals were grown by slow diffu-
sion of hexanes into concentrated dichloromethane solution of
complexes. An ORTEP plot of one molecule of 5 is presented in
Fig. 1. Complex 5 adopts a bipyramid trigonal geometry with
both iminophosphoranes and only one phosphine arm being
coordinated to the metal centre, the other one being remote
Fig. 2 Molecular structure of complex 6. Hydrogen atoms and solvent
molecules have been omitted for clarity. Selected distances (A) and angles
˚
from iron (5.729 A for the Fe–P4 distance). The Fe, N1, N2,
˚
and P2 atoms are almost coplanar with a small deviation of
4.52◦ from the planarity (N1–N2–Fe–P2 dihedral angle). The
coordination sphere is completed by two chloride atoms, the
(◦): P4–Fe = 5.046; N1–Fe = 2.018(2); N2–Fe = 2.019(2); P3–Fe = 5.045;
Fe–Cl2 = 2.301(7); Fe–Cl1 = 2.312(7); P1–N1 = 1.599(2); P2–N2 =
1.592(2); P3–S1 = 1.948(1); P4–S2 = 1.949(8); Cl1–Fe–Cl2 = 118.42(3);
Cl1–Fe–N2 = 113.88(6); Cl2–Fe–N2 = 110.46(6); P3–Fe–P4 = 156.88;
Cl1–Fe–N1 = 111.26(6); N1–Fe–N2 = 82.74(8); N1–Fe–Cl2 = 114.79(6);
N2–Fe–Cl2 = 110.46(6); N1–N2–P4–P3 = 12.98; P1–P2–P4–P3 = 8.46.
˚
Fe–Cl bonds measuring 2.406(7) and 2.350(7) A. As expected for
this d6 metal centre in a trigonal bipyramidal complex, a triplet
spin configuration is preferred for the d-block and justified the
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As can be seen the thiophosphine groups are rejected away
˚
from the metal centre, Fe–S distances being greater than 6.32 A.
The tetrahedral geometry around the d6 metal centre explains the
observed paramagnetism of complex 6. It is noteworthy that the
uncoordinated thiophosphine arms appeared deeply deshielded
compared to the free ligand (Dd ~ 19 ppm).
A view of complex 7 is presented in Fig. 3. As can be seen, com-
plex 7 adopts a square pyramidal geometry. The five coordination
sites being occupied by both iminophosphorane and phosphine
oxide arms, together with one chloride atom. Contrary to complex
6, complex 7 is monocationic, pentacoordinated and extremely
electron rich because of the coordination of the phosphine oxide
side arms. This might explain the greater sensitivity towards air
of 7 in comparison to 5 and 6. Indeed, in presence of oxygen, 7
instantly changes from yellow to red, being oxidized into iron(III),
whereas 5 and 6 could be isolated easily and handled for a few
minutes without any apparent oxidation.
Scheme 5 Synthesis of complex 8.
same complex, single crystals being grown in separate experiments.
A view of one molecule of complex 8 is presented in Fig. 4.
Fig. 4 Molecular structure of complex 8. Hydrogen atoms solvent
˚
molecules have been omitted for clarity. Selected distances (A) and
angles (◦): P4–Fe = 2.528(5); N1–Fe = 2.088(1); N2–Fe = 2.080(1);
P3–Fe= 2.483(5); Fe–Cl = 2.316(5); P1–N1 = 1.598(1); P2–N2 = 1.597(1);
P4–Fe–Cl = 95.20(2), P3–Fe–Cl = 100.82(2), N1–Fe–Cl = 109.42(4),
N2–Fe–Cl = 111.40(4), P3–Fe–P4 = 106.55(2); N1–Fe–N2 = 79.38(5);
P3–Fe–N1 = 82.21(4), P4–Fe–N2 = 79.42(4), N1–P3–P4–N2 = -4.59;
P1–P3–P4–P2 = 1.21.
Fig. 3 Molecular structure of complex 7. Hydrogen atoms solvent
molecules have been omitted for clarity. Selected distances (A) and
˚
angles (◦): O1–Fe1 = 2.132(2); N1–Fe1 = 2.097(2); N2–Fe1 = 2.100(2);
O2–Fe1 = 1.127(2); Fe1–Cl1 = 2.332(8); Fe1–Cl3 = 6.861; P1–N1 =
1.585(2); P3–N2 = 1.596(2); P4–O2 = 1.507(2), P2–O1 = 1.504(2),
O2–Fe1–O1 = 83.69(7); N1–Fe–N2 = 80.0(1); O2–Fe–N2 = 89.80(8);
O1–Fe–N1 = 90.78(1); O1–Fe–Cl1 = 107.89(6), O2–Fe–Cl1 = 99.23(6),
N1–Fe–Cl1 = 102.54(7), N2–Fe–Cl1 = 114.85(6), N1–O1–O2–N2 =
13.84; P1–P2–P4–P3 = 9.02.
As can be seen, 8 is a monocationic iron(II) complex, which
adopts a square pyramidal geometry. The five coordination sites
are occupied by both iminophosphorane and phosphine arms,
together with one chloride ligand. In comparison with complex 5,
all phosphines are strongly coordinated to the metal center
˚
(P4–Fe = 2.528(5) and P3–Fe = 2.483(5) A in 8 versus P2–Fe =
˚
˚
2.674 A in 5). The Fe–Cl distance was measured at 2.316(5) A.
Though the two phosphorus and the nitrogen atoms are almost
coplanar (dihedral angle N1–P3–P4–N2 = 4.59◦), the iron metal
centre is located above the square base of the pyramid (dihedral
angles P3–N1–N2–Fe = 22.81◦ and P4–N2–N1–Fe = 15.88◦).
In this strongly distorted square base pyramidal d6 complex of
a first period element, a triplet or a higher spin configuration
of the d block can be expected. Indeed, here the complex is
paramagnetic with a measured magnetic moment of 4.17 mB.
No additional experiments were undertaken to rationalize the
mechanism accounting for the formation of complex 8 but one
may propose that it results from a ligand exchange between co-
ligands (H2O and CH3CN) and chloride anions which are present
in solution.
In order to evaluate the potential of dicationic iron(II) complexes
in transfer hydrogenation reactions, we then logically attempted
the removal of chloride ligands from complex 5 using two
equivalents of AgBF4, but despite many efforts, whatever the
experimental conditions employed, this reaction failed. To obtain
the desired dicationic complex of general formula [Fe(P2N2)]2+,
coordination of ligand 2 was achieved using either the available
dicationic [Fe(H2O)6] (BF4)2 or [Fe(CH3CN)6](BF4)2 precursors.
Two different procedures were tested. In a first approach, both
iron precursors were added to a THF solution of free ligand 2
still containing LiCl salts resulting from the reaction with methyl
lithium (Scheme 5). However, no reaction yielded the expected
tetracoordinated dicationic species but the pentacoordinated
complex 8 (see Scheme 5). The structure of 8 could not be
established from NMR data (8 is 31P NMR silent) and definitive
evidence on its formulation was given by a X-ray crystal structure
analysis. Note that it was checked that both reactions yielded the
In order to circumvent this limitation further experiments were
carried out using LiCl-free solutions of ligand 2. Moreover,
an ionizing solvent such as acetonitrile was used. Under these
conditions (Scheme 6), addition of a stoichiometric amount of
[Fe(CH3CN)6](BF4)2 to an acetonitrile solution 2 induced an
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Table 1 Catalytic transfer hydrogenation of acetophenone
Entry
Complex
Reaction time/h
NMR conversion (%)
1a
2
3
4
5
6
8
5
6
7
8
9
18
8
8
6
8
48
80
91
89
75
78
8
a Reaction performed at 50 ◦C.
at 82 ◦C (propanol reflux), a minimal conversion of 75% was
observed in 1H NMR after 8 h, all reactions being completed after
24 h.
Scheme 6 Synthesis of complex 8 from 2 and from complex 5 by chloride
Comparing the different results, complex 7 appeared slightly
more efficient but, taken as a whole, the performances of the
complexes 5–9 were found to be comparable. Moreover, having
in mind that a cationic iron(II) hydride complex may be the
active catalytic species, the reaction was monitored by 1H{P}
NMR, but no putative iron hydride complex could be detected.
The synthesis of this iron hydride complex by reduction of 5
with sodium tetraborohydrate or lithium triethylborohydride in
THF was also attempted but did not yield convincing results,
each experiment resulting in the formation of many unidentified
species. Nevertheless iron hydride complexes are not always
powerful catalysts, since Bianchini’s group synthesised such a
complex, which only exhibited a low catalytic activity in transfer
hydrogenation of acetophenone.15 On the other hand, Casey
developed an elegant use of Kno¨lker’s iron hydride complex as
a racemic hydrogenation catalyst under mild conditions (room
temperature, low pressure of dihydrogen).
abstraction with GaCl3.
instant colour change from colourless to purple. Unfortunately,
monitoring the reaction by 31P{H} NMR did not bring significant
information on the nature of the compound formed since no signal
could be detected. However, a rapid decomposition was observed
as evidenced by the fading of the purple colour and the appearance
of many unidentified peaks in the 31P{H} NMR spectrum.
Anticipating that this decomposition may result from the high
reactivity of the putative transient [Fe(P2N2)(CH3CN)2](BF4)2
complex, two equivalents of tert-butylisocyanide (tBuNC) were
added to the purple solution as soon as it formed resulting in the
immediate appearance of an orange colour. The 31P{H} NMR
spectrum of the crude mixture also revealed the presence of an
A2B2 spin system pattern consisting of two virtual triplets centered
at (3JPP = 14.5 Hz) at d(CH3CN) = 43.2 and 65.2 ppm.‡ This
complex proved to be diamagnetic and was fully characterized
by multinuclear NMR spectroscopies and elemental analyses. It
is noteworthy that the same complex, labelled 9, was obtained
by chloride abstraction of complex 5 using GaCl3 followed
by addition of two equivalents of tert-butylisocyanide ligand.
Conventional work-up yielded a pale orange solid, which unfor-
tunately could not be further characterized by X-ray diffraction
since no suitable crystal could be obtained despite numerous
crystallization attempts. However, considering the coupling figures
obtained in the NMR spectrum, we propose that 9 adopts a
pseudo-octahedral geometry as depicted in Scheme 6.
To complete this study, complexes 5, 8 and 9 were also tested
as catalytic precursors for the hydrogenation of acetophenone to
1-phenylethanol under H2 atmosphere. The results were found to
be disappointing since no reaction was observed for 5 and 9 after
20 h at 60 ◦C. With 8 the conversion only reached 10%.
While ruthenium-based systems for the transfer hydrogenation
of ketones are well known in the literature,16 examples of active
iron-based systems are still very rare. Aside from the system
reported by Morris’s group at the beginning of 2008, which turns
out to be the most efficient reported so far (room temperature
activity, good enantioselective excess),9 complexes 5–9 can be con-
sidered as quite competitive, though racemic and requiring higher
temperature. At 0.1% molar catalyst loading, these complexes
compete favorably with the racemic system developed by Beller
in 2006 based ◦on iron sources and porphyrins (1% molar of iron,
50% base, 100 C, 90% conversion of acetophenone in 7 h).17 Even
Casey’s catalyst required higher catalyst loading (1% molar) for
the racemic transfer hydrogenation reaction, 87% of acetophenone
being converted in 1-phenylethanol within 16 h at 75 ◦C.
B
Catalytic transfer hydrogenation of acetophenone
Complexes 5–9 were evaluated in the catalytic transfer hydro-
genation of acetophenone in isopropanol using 4% of base and
a low catalyst loading of 0.1%. Different bases were tested
(KOH, t-BuOK, and i-PrONa); no significant difference being
observed. Reactions were conducted with sodium isopropanolate.
The results are gathered in Table 1. No reaction took place at room
temperature, and the evolution was only very sluggish at 50 ◦C
(48% conversion after 18 h with complex 8, entry 1). However
Conclusions
In summary, we have developed a series of well-defined iron
catalyst based on a family of readily accessible mixed tetradentate
‡ When only one equivalent of isocyanide was added, a mixture of complex
9 and the transient [(P2N2)Fe+] complex was observed.
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3
3
ligands bearing two iminophosphorane moieties and either two
phosphine, thiophosphine or phosphine oxide groups. All those
ligands are easily synthesised from the same bis(phosphine-
aminophosphonium) salt, prepared from commercial reagents.
The variety of the coordination sites in this family of ligands
accounts for the structural diversity of the prepared iron(II)
complexes 5–9. All present different geometries: trigonal bipyra-
midal, pseudo-octahedral, square based pyramidal, or tetra-
hedral. Moreover, neutral, monocationic, and dicationic iron
species were prepared using the same tetradentate phosphine-
iminophosphorane ligand 2 by simply modifying the nature of the
iron(II) precursor. Most of these complexes were paramagnetic
and were characterized by X-ray diffraction analysis. All these
complexes constitute well-defined iron catalysts for the transfer
hydrogenation of acetophenone. Active at 0.1% mol, those precat-
alysts give satisfactory TON (1000) and TOF (150 h-1 for complex
7) and are among the best iron catalysts reported so far for the
transfer hydrogenation of ketones. Mechanistic studies and the
development of pure enantiomeric derivatives of these new systems
are currently underway in our laboratories.
p-H (Ph2P(V)(N)), 7.77 (8H, dd, JHH = 7.6 Hz, JHP = 13.4 Hz,
o-H (Ph2P(V)(N)), 7.87 (8H, dd, 3JHH = 7.0 Hz, 3JHP = 14 Hz, o-H
Ph2P(V)(S), NH not seen. 13C { H} (75.5 MHz, CDCl3) dC 27.3
1
1
1
2
(dd, JCP = 49.0 Hz, JCP = 66.5 Hz, PCH2P), 40.5 (d, JCP
=
8.5 Hz, NCH2), 118.3 (d, JCP = 100.0 Hz Cipso-(Ph2 P(V))), 127.7
(d, 3JCP = 13.1 Hz, m-CH-(Ph2P(V)(S)), 128.1 (d, 3JCP = 13.6 Hz,
m-CH-(Ph2P(V)(N)), 130.1 (d, 2JCP = 11.4 Hz, o-CH-(Ph2P(V)(S)),
2
130.4 (dd, JCP = 81.8 Hz, JCP = 3.5 Hz, Cipso-(Ph2 P(V))), 130.6
1
3
(d, JCP = 2.7 Hz, p-CH-(Ph2P(V)(S)), 133.3 (d, JCP = 11.5 Hz,
4
2
o-CH-(Ph2P(V)(N)), 133.7 (d, JCP = 2.7 Hz, p-CH-(Ph2P(V)(N)).
4
Anal. Calcd. for C52H50Cl2N2P4S2 : C, 64.93; H, 5.24; N, 2.91%.
Found: C, 64.87; H, 5.21; N, 2.94%.
Synthesis of phosphine oxide/aminophosphonium salt 4-(HCl)2.
A solution of H2O2 in water (35% wt) (74 mL, 0.84 mmol) was
slowly (the reaction is exothermic) added to a suspension of 2-
(HCl)2 (500 mg, 0.557 mmol) in dichloromethane (5 mL). The
suspension was stirred for 5 min at room temperature. The solution
was washed twice with water (5 mL), the organic layer was dried
over MgSO4 and the solvent was removed under vacuum to deliver
a white solid, which was washed with diethyl ether.
4-(HCl)2 (440 mg, 85%).31P { H} (121.5 MHz, CDCl3) d 28.7
1
Experimental section
(d, JPP = 7.6 Hz, P(V)(O), 44.6 (d, JPP = 7.6 Hz, P(V)(N). H
2
2
1
(300 MHz, CDCl3) d 3.20 (4H, bs, N-CH2), 4.88 (4H, dd, 2JHP
=
General considerations
12.7 Hz, 2JHP = 17.0 Hz, PCH2P), 7.28 (8H, m, p,m-H (Ph2 P(V)(O)),
7.33 (8H, m, m-H (Ph2P(V)(N)), 7.49 (8H, t, JHH = 7.5 Hz, p-H
3
All experiments were performed under an atmosphere of dry
nitrogen or argon using standard Schlenk and glove box tech-
niques. Solvents were freshly distilled under dry nitrogen from
Na/benzophenone (THF, diethylether, petroleum ether), from
P2O5 (dichloromethane). Aminophosphonium salt 2-(HCl)2,10
sodium isopropanolate, [Fe(MeCN)6](BF4)218 and [FeCl2THF1.5]19
were prepared according to literature procedures. All other
reagents and chemicals were obtained commercially and used
without further purification, except for isopropanol which was
distilled under dry nitrogen from calcium hydride. Nuclear mag-
netic resonance spectra were recorded on Bruker Avance 300
(Ph2P(V)(N)), 7.82 (8H, dd, JHH = 7.5 Hz, JHP = 12.6 Hz, o-H
3
3
(Ph2P(V)(O)), 7.90 (8H, dd, JHH = 7.9 Hz, JHP = 13.7 Hz, o-
3
4
H (Ph2P(V)(N)), NH not seen. 13C { H} (75.5 MHz, CDCl3) dC
1
28.0 (dd, 1JCP = 56.6 Hz, 1JCP = 60.0 Hz, PCH2P), 42.2 (d, 2JCP
=
9.0 Hz, NCH2), 120.7 (d, 1JCP = 100.6 Hz, Cipso-(Ph2 P(V)(O)), 129.0
(d, 3JCP = 12.8 Hz, m-CH-(Ph2 P(V)(O)), 129.6 (d, 3JCP = 13.7 Hz,
m-CH-(Ph2 P(V)(N)), 131.0 (d, 3JCP = 10.5 Hz, o-CH-(Ph2P(V)(N)),
132.2 (dd, 1JCP = 100.6 Hz, 3JCP = 3.2 Hz, Cipso-(Ph2P(V)(N)), 132.4
(d, JCP = 2.7 Hz, p-CH-(Ph2P(V)(O)), 134.0 (d, JCP = 12.0 Hz,
2
2
o-CH-(Ph2P(V)(N)), 134.8 (d, JCP = 2.8 Hz, p-CH-(Ph2P(V)(N)).
2
1
spectrometer operating at 300 MHz for H, 75.5 MHz for 13C
Anal. Calcd. for C52H50Cl2N2P4O2 : C, 67.17; H, 5.42; N, 3.01%.
Found: C, 67.13; H, 5.39; N, 3.06%.
and 121.5 MHz for 31P. 1H and 13C chemical shifts are reported in
ppm relative to Me4Si as external standard. 31P chemical shifts are
relative to a 85% H3PO4 external reference. Coupling constants
are expressed in hertz. The following abbreviations are used: b,
broad; s, singlet; d, doublet; dd, doublet of doublets; t, triplet; m,
multiplet; v, virtual. Magnetic moments were determined by the
Evans method13 in CDCl3 using a solution of Me4Si in CDCl3
(1 : 100, V/V) as reference. Elemental analyses were performed by
the “Service d’analyse du CNRS”, at Gif sur Yvette, France.
Synthesis of thiophosphine/iminophosphorane 3. MeLi
(130 mL, 0.208 mmol) was added to a suspension of ligand
3-(HCl)2 (100 mg, 0.104 mmol) in toluene (5 mL) cooled
at-78 ◦C. The cold bath was removed and the solution allowed
to warm to room temperature. The insoluble lithium salts were
centrifuged from the yellow solution and after removal of solvent
under vacuum, the obtained white solid was washed with hexanes
(10 mL).
3 (76 mg, 76%).31P { H} (121.5 MHz, C7D8) d 33.2 (d, JPP
=
1
2
Syntheses
26.2 Hz, P(V)(N)), 34.8 (d, 2JPP = 26.2 Hz, P(V)(S)). 1H (300 MHz,
C7D8), 3.12 (4H, bs, N-CH2), 4.59 (4H, d, 2JHP = 6.5 Hz, PCH2P),
7.19–7.35 (24H, m, p,m-H (Ph2 P(V))), 7.79 (8H, dd, 3JHH = 7.3 Hz,
Synthesis of thiophosphine/aminophosphonium salt 3-(HCl)2.
S8 (36 mg, 0.140 mmol) was added to a solution of 2-(HCl)2
(500 mg, 0.557 mmol) in dichloromethane (5 mL). The reaction
mixture was stirred for 30 min at 40 ◦C and a white precipitate
was rapidly formed. The solvents were removed under vacuum to
yield a white solid, which was washed with diethylether.
3JHP = 12.0 Hz, o-H (Ph2P(V)(S)), 7.93 (8H, m, o-H (Ph2P(V)(N)).
13C { H} (75.5 MHz, C7D8) dC 12.0 (dd, 1JCP = 106.8 Hz, 1JCP
135.5 Hz, PCH2P), 41.2 (d, 2JCP = 8.1 Hz, NCH2), 126.7 (d, 3JCP
=
=
1
12.0 Hz, m-CH-(Ph2P(V)),127.6 (d, 3JCP = 12.4 Hz, m-CH-(Ph2P(V)),
128.5 (d, 4JCP = 2.3 Hz, p-CH-(Ph2P(V)), 130.0 (d, 2JCP = 10.7 Hz, o-
CH-(Ph2P(V)(S)), 130.4 (d, 4JCP = 2.1 Hz, p-CH-(Ph2P(V)(N)), 131.4
(d, 2JCP = 10.0 Hz, o-CH-(Ph2P(V)(S)), 133.7 (d, 4JCP = 2.7 Hz, p-
1
3-(HCl)2 (457 mg, 92%). 31P { H} (121.5 MHz, CDCl3) d 37.0
(s, P(V)(S), 44.8 (s, P(V)(N). 1H (300 MHz, CDCl3) d 2.79 (4H, bs,
2
2
N-CH2), 5.03 (4H, dd, JHP = 13.2 Hz, JHP = 3.4 Hz, PCH2P),
7.27 (20H, m, p-H (Ph2P(V)(S) and m-H (Ph2P(V)(N)), 7.46 (4H, m,
CH-(Ph2P(V)(N)), 132.8 (dd, JCP = 109.2.0 Hz, JCP = 5.6.0 Hz,
1
4
1664 | Dalton Trans., 2009, 1659–1667
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1
4
Cipso-(Ph2P(V))), 141.9 (dd, JCP = 84.4 Hz, JCP = 4.1 Hz, Cipso
-
Synthesis of complex [P2N2FeCl]BF4 8. MeLi (140 mL,
0.222 mmol) was added to a suspension of ligand 2-(HCl)2
(0.111 mmol) in THF (5 mL) cooled at -78 ◦C. The cold bath was
removed and the solution allowed to warm to room temperature.
Then, [Fe(BF4)2MeCN6)] (53 mg, 0.111 mmol) was added and
the solution turned immediately from colourless to yellow. After
stirring 30 min at room temperature, THF was evaporated, the
resulting solid was dissolved into CH2Cl2 (5 mL) to remove the
insoluble lithium salts. After evacuation of solvents under vacuum,
the obtained very air-sensitive yellow solid was washed with Et2O
(10 mL) and then dried. Crystals suitable for X-ray diffraction
were obtained by slow diffusion of petroleum ether into a solution
of 8 in dichloromethane at room temperature. 8 (77 mg, 69%).1H
(300 MHz, CDCl3) d 0.89 (4H, bs, N-CH2), 4.65 (4H, bs, PCH2P),
7.0–7.74 (40H, m, (Ph2P). meff (Evans balance): 4.17 mB. Anal.
Calcd. for C52H48BClF4FeN2P4 : C, 62.27; H, 4.82; N, 2.79%.
Found: C, 62.31; H, 4.85; N, 2.78%.
(Ph2P(V))).
Synthesis of phosphine oxide/iminophosphorane 4. MeLi
(135 mL, 0.215 mmol) was added to a suspension of ligand 4-(HCl)2
(100 mg, 0.107 mmol) in toluene (5 mL) cooled at -78 ◦C. The
cold bath was removed and the solution allowed to warm to
room temperature. The insoluble lithium salts were removed by
centrifugation from the colourless solution. After removal of
solvent under vacuum, the obtained white solid was washed with
hexanes (10 mL).
4 (73 mg, 82%).31P { H} (121.5 MHz, C7D8) d 27.9 (d, JPP
=
1
2
12.0 Hz, P(V)(N)), 38.7 (d, 2JPP = 12.0 Hz, P(V)(O)). 1H (300 MHz,
C7D8) d 1.78 (4H, bs, PCH2P), 2.64 (4H, bs, N-CH2), 7.18 (4H, m,
p-H (Ph2 P(V)O), 7.25 (16H, m, m, o-H (Ph2P(V)(N), 7.46 (4H, m, o-
H (Ph2P(v)O). 13C { H} (75.5 MHz, C7D8) dC 36.5 (t, 1JCP = 94.0 Hz,
1
1JCP¢ = 135.5 Hz, PCH2P), 49.5 (d, 2JCP = 16.0 Hz, NCH2), 126.7
(d, JCP = 6.0 Hz, p-CH-(Ph2P(V)(O)), 127.7 (d, JCP = 10.0 Hz,
3
3
o-CH-(Ph2P(V)(O), 130.5 (d, JCP = 5.0 Hz, p-CH-(Ph2P(V)(N)),
2
Synthesis of complex [P2N2Fe(tBuNC)2](BF4)2 9. MeLi
(140 mL, 0.222 mmol) was added to a suspension of ligand 2-
(HCl)2 (0.111 mmol) in toluene (5 mL) cooled at -78 ◦C. The
cold bath was removed and the solution allowed to warm to room
temperature. After removal of the insoluble lithium salts from the
colourless solution and evaporation of solvents under vacuum,
acetonitrile (5mL) was added. Then, [Fe(BF4)2MeCN6)] (53 mg,
0.111 mmol) was introduced, the solution turning immediately
from colourless to purple. The immediate addition of tBuNC
(0.026 mL, 0.222 mmol) induced a rapid colour change to
orange. After stirring 30 min at room temperature, acetonitrile
was evaporated and the resulting air stable orange solid was
131.0 (d, JCP = 9.0 Hz, m-CH-(Ph2P(V)(O)), 131.8 (d, JCP
=
4
2
9.0 Hz, m-CH-(Ph2P(V)(N)), 132.1 (d, JCP = 10.0 Hz, o-CH-
4
(Ph2P(V)(O)), 139.8 (d, JCP = 36.5 Hz, Cipso-(Ph2P(V))), 141.7 (d,
1
1JCP = 30.5 Hz Cipso-(Ph2P(V))).
Synthesis of complex [P2N2FeCl2] 5. MeLi (140 mL,
0.222 mmol) was added to a suspension of ligand 4-(HCl)2
(0.111mmol) in THF (5 mL) cooled at -78 ◦C. The cold bath was
removed and the solution allowed to warm to room temperature.
Then, [FeCl2THF1.5)] (26 mg, 0.111 mmol) was added and the
solution turned immediately from colourless to yellow. After
stirring 30 min at room temperature, THF was evaporated
and the resulting solid was dissolved into CH2Cl2 (5 mL) to
remove the insoluble lithium salts. After evaporation of solvents
under vacuum, the obtained very air-sensitive yellow solid was
washed with Et2O (10 mL) then dried. Crystals suitable for X-ray
diffraction were obtained by slow diffusion of petroleum ether into
a solution of 5 in dichloromethane at room temperature. 5 (83 mg,
1
washed with Et2O (10 mL) then dried. 9 (100 mg, 74%).31P { H}
2
(121.5 MHz, C7D8) d 43.2 (vt, JPP = 14.5 Hz, P(V)(N)), 65.2
2
1
(vt, JPP = 14.5 Hz, P(III)). H (300 MHz, C7D8) d 0.99 (18H, s,
(CH3)3-C), 3.02 (4H, bs, N-CH2), 3.89 (4H, bs, PCH2P), 7.00 (4H,
m, p-H (Ph2P(III))), 7.16 (8H, m, o,m-H (Ph2P(III))), 7.33 (8H, m,
o,m-H (Ph2P(III))), 7.45 (8H, m, o,m-H (Ph2P(V))), 7.56(4H, m, p-
1
H (Ph2P(V))), 7.61 (8H, m, o,m-H (Ph2P(V))). 13C { H} (75.5 MHz,
78%).31P { H} (121.5 MHz, CDCl3) d -27.0 (bs, non coordinated
1
C7D8) dC 29.5 (s, (CH3)3-C), 35.7 (d, 1JCP = 85.0 Hz), 51.6 (d, 2JCP
=
P(III)). 1H (300 MHz, CDCl3) d 4.21 (4H, bs, N-CH2), 4.99 (4H, bs,
PCH2P), 7.45 (16H, bs, H (Ph2 P), 7.77 (8H, bs, H (Ph2 P), 8.97
(6H, m, H (Ph2P)). meff (Evans balance): 4.36 mB. Anal. Calcd. for
C53H50Cl2FeN2P4: C, 65.92; H, 5.22; N, 2.90%. Found: C, 66.03;
H, 5.29; N, 2.88%.
13.0 Hz, NCH2), 59.1 (Cipso-(CH3)3), 127.6 (d, 1JCP = 87.0 Hz, Cipso
-
(Ph2P(III))), 129.0 (t, JCP = 5.0 Hz, CH-(Ph2P)), 129.7 (d, JCP
=
12.0 Hz, CH-(Ph2P)), 130.9 (d, JCP = 10.0 Hz, CH-(Ph2P)), 131.6
(s, CH-(Ph2P)), 132.5 (d, JCP = 11.0 Hz, CH-(Ph2P)), 132.6 (t,
1
4
JCP = 5.0 Hz, CH-(Ph2P)), 133.8 (dd, JCP = 26.0 Hz, JCP
=
Synthesis of complex [S2N2FeCl2] 6. The preparation of 6
(yielded as a very air-sensitive light brown solid) and the obtain-
ment of crystals were carried out using the same methods already
2.5 Hz, Cipso-(Ph2P(V))), 133.9 (s, CH-(Ph2P)), d Cipso-(CH3)3 not
measurable. Anal. Calcd. for C62H66B2F8FeN4P4 : C, 61.01; H,
5.45; N, 4.59%. Found: C, 61.09; H, 5.48; N, 4.62%.
described for 5. 6 (95 mg, 85%).31P { H} (121.5 MHz, CDCl3) d 57
1
(bs, not coordinated P(V)(S). 1H (300 MHz, CDCl3) d 2.18 (4H, bs,
N-CH2), 4.40 (4H, bs, PCH2P), 7.29 (24H, m, H (Ph2P)), 7.85 (8H,
m, H (Ph2P)), 10.56 (8H, m, o-H (Ph2 P)). meff (Evans balance):
3.87 mB. Anal. Calcd. for C52H47Cl2FeN2P4S2: C, 61.55; H, 4.67;
N, 2.76%. Found: C, 61.59; H, 4.78; N, 2.73%.
General procedure for the transfer hydrogenation of acetophenone
A Schlenk was charged under nitrogen atmosphere with the iron
complex (0.01 mmol, 9.5 mg for 5, 10.2 mg for 6, 9.8 mg for 7,
10.0 mg for 8) and sodium isopropanolate (32.8 mg, 0.4 mmol).
Isopropanol (5 mL) and acetophenone (1.15 mL, 10 mmol) were
then added and the reaction mixture was vigorously stirred under
isopropanol reflux (82 ◦C). The progress of the reaction was
monitored by 1H NMR. At the appropriate time, an aliquot was
taken from the reaction mixture, dried under vacuum, flushed on
a short silica column with CDCl3 to remove the iron complexes,
and then analysed by 1H NMR.
Synthesis of complex [O2N2FeCl2] 7. The preparation of 7
(yielded as a very air-sensitive brown solid) and the obtainment of
crystals were carried out using the same methods already described
for 5. 7 (73 mg, 80%).1H (300 MHz, CDCl3) d 0.81 (4H, bs, N-
CH2), 1.18 (4H, bs, PCH2P), 7.18 (40H, bs, (Ph2P)). meff (Evans
balance): 4.03 mB. Anal. Calcd. for C52H47Cl2FeN2O2P4: C, 63.56;
H, 4.82; N, 2.85%. Found: C, 63.61; H, 4.89; N, 2.79%.
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Table 2 Crystal data and structural refinement details for 5, 6, 7 and 8
Compound
P2N2FeCl2 5
S2N2FeCl2 6
O2N2FeCl2 7
[P2N2FeCl]BF4 8
Crystal size/mm
Crystal habit
Empirical formula
0.40 ¥ 0.24 ¥ 0.18
Pale yellow block
C52H48Cl2FeN2P4, 1.5
C4H8O, CH2Cl2
1144.64
Monoclinic
C2/c
36.269(1)
15.077(1)
19.211(1)
90.00
90.424(1)
90.00(1)
10504.8(9)
8
1.447
0.659
28.70
150.0(1)
4768
0.30 ¥ 0.20 ¥ 0.10
Pale yellow block
C52H46Cl2FeN2P4S2, 3
CH2Cl2
1268.44
Triclinic
¯
P1
13.700(1)
15.476(1)
15.866(1)
91.952(1)
114.158(1)
103.426(1)
2953.7(3)
2
1.426
0.834
30.03
150.0(1)
1300
0.22 ¥ 0.16 ¥ 0.14
Pale yellow block
C52H48ClFeN2O2P4,Cl,
CH2Cl2
1153.41
Triclinic
¯
P1
11.611(1)
20.231(1)
25.508(1)
108.182(1)
102.266(1)
92.630(1)
5522.0(6)
4
1.387
0.721
27.47
150.0(1)
0.38 ¥ 0.36 ¥ 0.26
Pale yellow block
C52H48ClFeN2P4,CH2Cl2,
BF4
1087.84
Triclinic
¯
P1
10.868(1)
13.934(1)
18.166(1)
100.819(1)
101.043(1)
102.516(1)
2559.3(3)
2
1.412
0.630
30.02
150.0(1)
Molecular mass
Crystal system
Space group
˚
a/A
˚
b/A
˚
c/A
a/◦
b/◦
g /◦
3
˚
V/A
Z
Calcd density/g cm-3
Abs. coefficient/cm-1
◦
qmax
T/K
F(000)
Index ranges
/
2376
1120
-48–38; -20–18; -25–22
34140/13547
9652
-19–19; -21–21; -22–22
39926/17238
12502
-15–14; -26–24; 0–33
24439/24439
17218
-15–15; -18–19; -25–25
29603/14716
11542
Refl. collected/independant
Refl. used
(Rint
)
0.0337
0.7786 min., 0.8907 max.
0.0285
0.7879 min., 0.9212
max.
0.0364
0.8575 min, 0.9058 max
0.0247
0.7959 min., 0.8534 max.
Abs. corr.
Parameters refined
573
16
649
19
1142
15
0.0545/0.1608
1.092
0.820/-0.765
613
14
Reflection/parameter
Final R1/wR2 [I>2sI)]
Goodness-of-fit on F2
0.0552/0.1840
1.100
1.499/-0.642
0.0486/0.1422
1.066
1.451/-0.917
0.0394/0.7764
1.062
0.816/-0.809
-3
˚
Difference peak/hole/e A
General procedure for the hydrogenation of acetophenone
Acknowledgements
The reaction was performed in a magnetically stirred 120 mL
stainless steel autoclave, equipped with a pressure gauge. A
solution of the iron complex (0.01 mmol, 9.5 mg for 5, 10.0 mg for
8) and acetophenone (1.15 mL, 10 mmol) in toluene (20 mL) was
introduced in the reactor under nitrogen atmosphere. The reactor
was then charged with H2 (20 bar) and brought to the working
temperature (80 ◦C). After 24 h, the autoclave was allowed to
cool down to room temperature then depressurized. The reaction
mixture was dried up under vacuum, flushed on a short silica
column with CDCl3 to remove the iron products, then analysed by
1H NMR.
We thank the CNRS, the Ecole Polytechnique, and the ANR (Pro-
gramme blanc ANR-06-BLAN-0060-01) for financial support.
Notes and references
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X-Ray crystallography
Data were collected on a Nonius Kappa CCD diffractometer
˚
using a Mo-Ka (l = 0.71073 A) X-ray source and a graphite
monochromator. Experimental details are described in Table 2.
Electronic supplementary information contains the supplemen-
tary crystallographic data for this paper (CIF files for complexes
5, 6, 7 and 8). CCDC reference numbers 702897–702900. For
crystallographic data in CIF or other electronic format see DOI:
10.1039/b816439h. The crystal structure was solved using SIR
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